1. Field of the Invention
The present invention relates to compounds and compositions of cytosine analogs and derivatives, and methods for preparing, formulating and administering these compounds or compositions to a host in need thereof.
2. Description of Related Art
A few nucleosides of azacytosine, such as 5-aza-2′-deoxycytidine (also called decitabine, FIG. 1A, structure 1a) and azacitidine (FIG. 1A, structure 1b), have been developed as antagonist of its related natural nucleoside, cytidine and 2′-deoxycytidine, respectively. The only structural difference between azacytosine and cytosine is the presence of a nitrogen atom at position 5 of the cytosine ring in azacytosine as compared to a carbon at this position for cytosine. Decitabine (1a) was prepared by cyclization of peracylglycosyl isocyanates (Pliml and Sorm (1964) Collect. Czech. Chem. Commun. 29:2576–2577); later azacitidine (1b) and a series of related analogues were prepared (Naeem et. al. (1998) Collect. Czech. Chem. Commun. 63:222–230).
Two isomeric forms of decitabine can be distinguished. The β-anomer is the active form. The modes of decomposition of decitabine in aqueous solution are (a) conversion of the active β-anomer to the inactive α-anomer (Pompon et al. (1987) J. Chromat. 388:113–122); (b) ring cleavage of the aza-pyrimidine ring to form N-(formylamidino)-N′-β-D-2′-deoxy-(ribofuranosy)-urea (Mojaverian and Repta (1984) J. Pharm. Pharmacol. 36:728–733); and (c) subsequent forming of guanidine compounds (Kissinger and Stemm (1986) J. Chromat. 353:309–318).
Decitabine possesses multiple pharmacological characteristics. At a molecular level, it is S-phase dependent for incorporation into DNA. At a cellular level, decitabine can induce cell differentiation and exert hematological toxicity. Despite having a short half life in vivo, decitabine has an excellent tissue distribution.
One of the function of decitabine is its ability to specifically and potently inhibit DNA methylation. Methylation of cytosine to 5-methylcytosine occurs at the level of DNA. Inside the cell, decitabine is first converted into its active form, the phosphorylated 5-aza-deoxycytidine, by deoxycytidine kinase which is primarily synthesized during the S phase of the cell cycle. The affinity of decitabine for the catalytical site of deoxycytidine kinase is similar to the natural substrate, deoxycytidine.
Momparler et al. (1985) 30:287–299. After conversion to its triphosphate form by deoxycytidine kinase, decitabine is incorporated into replicating DNA at a rate similar to that of the natural substrate, dCTP. Bouchard and Momparler (1983) Mol. Pharmacol. 24:109–114.
Incorporation of decitabine into the DNA strand has a hypomethylation effect. Each class of differentiated cells has its own distinct methylation pattern. After chromosomal duplication, in order to conserve this pattern of methylation, the 5-methylcytosine on the parental strand serves to direct methylation on the complementary daughter DNA strand. Substituting the carbon at the 5 position of the cytosine for a nitrogen interferes with this normal process of DNA methylation. The replacement of 5-methylcytosine with decitabine at a specific site of methylation produces an irreversible inactivation of DNA methyltransferase, presumably due to formation of a covalent bond between the enzyme and decitabine. Juttermann et al. (1994) Proc. Natl. Acad. Sci. USA 91:11797–11801. By specifically inhibiting DNA methyltransferase, the enzyme required for methylation, the aberrant methylation of the tumor suppressor genes can be prevented.
Decitabine is commonly supplied as a sterile lyophilized powder for injection, together with buffering salt, such as potassium dihydrogen phosphate, and pH modifier, such as sodium hydroxide. For example, decitabine is supplied by SuperGen, Inc., as lyophilized powder packed in 20 mL glass vials, containing 50 mg of decitabine, monobasic potassium dihydrogen phosphate, and sodium hydroxide. When reconstituted with 10 mL of sterile water for injection, each mL contain 5 mg of decitabine, 6.8 mg of KH2PO4, and approximately 1.1 mg NaOH. The pH of the resulting solution is 6.5–7.5. The reconstituted solution can be further diluted to a concentration of 1.0 or 0.1 mg/mL in cold infusion fluids, i.e., 0.9% Sodium Chloride; or 5% Dextrose; or 5% Glucose; or Lactated Ringer's. The unopened vials are typically stored under refrigeration (2–8° C.; 36–46° F.), in the original package.
Decitabine is most typically administrated to patients by injection, such as by a bolus I.V. injection, continuous I.V. infusion, or I.V. infusion. The length of I.V. infusion is limited by the decomposition of decitabine in aqueous solutions.
Thus, a need still exists for compounds, compositions and methods for improving chemical stability of cytosine analogs, especially in aqueous solutions. The present invention provides such improvements.